I been trying to justify replacing the main board as a last-ditch effort, I'll try it. The freezer and fridge thermistor both reported accurate temperatures in the service mode checks. Here is another symptom: This first happened a month ago. I looked for bad electrical connections, replaced the start relay and even the freezer thermostat. The unit started up and ran fine for 3 weeks. I suspected at the time that I hadn't really "fixed" anything, that the mere passage of time might have had more to do with the refrigerator resuming normal operation than anything I did. So does it say anything that after the unit was off for 4 or 5 days it was able to resume functioning for a few weeks? Does that help confirm any of your other diagnoses?

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It is also possible there is a bad connection to the compressor when it gets warm. Get a meter and monitor the compressor voltage. When it is cold, voltage to it should be 115 volts, when hot, should be the same when it tries to start.

Replaced both thermistors. Unit cooled to proper temps, shut off, then 10 minutes later the compressor began its regular routine of trying to start (relay buzzing) stopping for a couple of minutes, and trying again, and so on. Here is a new and curious symptom: After the compressor tried and failed to start a few times I turned off the unit, unplugged the wiring harness to the relay, turned the unit back on with a volt meter on the harness to see if it was still getting 120v (all OK) turned the unit off, plugged the harness back in, turned the unit on, compressor started right up EVEN THOUGH HOT!? With the volt meter in place I had jiggled every wire in sight trying to see if I could get a voltage drop, nothing, a nice steady 120, so I don’t think that I inadvertently made a better connection when reattaching the harness. A lose connection is something I looked for right from the start, and wiggling the wiring harness to the relay, pressing in on it, etc.,, when the compressor was trying to start all had no effect.
I went the through a similar routine a second time when the compressor failed to start hot, omitting only the attaching of the voltmeter: turned the unit off, disconnected harness to relay, turned the unit on, turned it off again, attached harness, turned unit on: hot compressor started up. VERY strange to me, any help to you?

Follow up: When the compressor failed to restart again after cooling the freezer/fridge to proper temps, I went through the above routine again with no effect: hot compressor did not start. I'm baffled....

I only have a very cheap and likely inaccurate amp meter. But when the compressor successfully started it seemed to draw about 9.5 amps, and when it failed to start (i.e., while the relay was buzzing) the meter peaked about 7.3-4 amps.

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